This invention relates to a light adjusting circuit for adjusting the luminous energy (or quantity of light) of a light-emitting element such as a laser diode or a light-emitting diode used in an optical device such as an optical disk device for CD or LD, a laser printer or an optical communication device.
A prior art light adjusting circuit, used in an optical disk device or the like which requires the luminous energy of its light-emitting element such as a laser diode or a light-emitting diode to be kept at a constant level, may be characterized, as shown in FIG. 4, as comprising a constant current source 3 for supplying a current to a light-emitting element 1 such as a light-emitting diode of which the luminous energy is required to remain constant, a light-receiving element 2 such as a photo-diode, a current-to-voltage conversion circuit 11 for converting the current generated in the light-receiving element 2 into a voltage, an amplifier circuit 12 for amplifying the converted voltage and a sample-and-hold circuit 13 for holding an analog voltage at a given time. As the light emitted from the light-emitting element 1 is received by the light-receiving element 2, there is generated an electric current, of which the intensity depends on the luminous energy received thereby. After this current is passed through the conversion circuit 11 to be converted into a voltage and the voltage thus obtained by conversion is amplified by the amplifier 12, this amplified voltage is inputted to the sample-and-hold circuit 13 which serves to sample it at a selected timing and to hold it as an analog voltage value representing the level of luminous energy. At the same time, the current from the constant current source 3 is reset according to the voltage held in the sample-and-hold circuit 13 such that the luminous energy of the light-emitting element 1 will be kept at a constant level.
The light adjusting circuit shown in FIG. 4 is advantageous in that its circuit structure is relatively simple. Because the voltage value in the sample-and-hold circuit 13 is held only as a voltage by a capacitor belonging to the sample-and-hold circuit 13, however, there may arise problems due to the leak current through the switch circuit or the capacitor within the sample-and-hold circuit 13. Because of such a leak current, for example, the voltage held by the sample-and-hold circuit 13 drops gradually (or "drift"), causing the current intensity of the constant current source 3 as well as the luminous energy of the light-emitting diode 1 to change gradually. When the device is operated in a high-temperature environment, in particular, the leak current through the switch circuit increases and the variations in the voltage value held in the sample-and-hold circuit 13 and the luminous energy become even greater. Since the input impedance of the sample-and-hold circuit 13 is high when its switch circuit is opened, there is a further problem that the voltage value held therein is susceptible to sudden changes due to the external noise invading the circuitry.
In view of the problems of the kind described above, it is sometimes necessary, when a prior art light adjusting circuit is used in an optical disk device or the like, to include a program for adjusting the change in luminous energy due to a leak current at several places in the control program for the optical disk device. This makes the control program complicated for the device, and high-speed operations of the device becomes difficult. In order to reduce the variations in the voltage values held in the sample-and-hold circuit due to the leak current through the capacitor, furthermore, it is necessary either to increase the capacitor area on an semiconductor element to thereby increase its capacitance or to attach a capacitor with large capacitance externally. This, however, goes against the general effort to include the light adjusting circuit inside a semiconductor integrated circuit or to reduce the number of externally attached components such that luminous energy can be adjusted easily and inexpensively. Moreover, since the capacitor is discharged and the voltage value being held will change whenever the source voltage is switched off, the information stored before the source is cut off cannot be used when the source is switched on again. This makes it necessary, whenever the source voltage is switched on, to detect the level of luminous energy and to thereby set the voltage to be held. This causes a delay in the operation of the device whenever the source is switched on.